Atomic Test And Set Of Disk Block Returned False For Equality May 2026

Why would the equality test fail? Usually, it's one of three scenarios: 1. "Split Brain" or Multi-Host Contention

In the world of distributed systems, high-availability clusters, and storage area networks (SANs), data integrity is the highest priority. One of the most cryptic yet significant errors a systems administrator or storage engineer might encounter is:

The "atomic test and set of disk block returned false for equality" error is a protective measure. While it causes disruptive downtime, it exists to prevent the "silent killer" of enterprise computing: By failing the operation when the state doesn't match, the system ensures that two hosts never write to the same block simultaneously, preserving the integrity of your databases and virtual machines. Why would the equality test fail

If the host cannot "set" the lock, it cannot write to the disk.

The VMkernel logs will fill with ATS Miscompare or Status: Op: 0x89 messages. How to Troubleshoot and Fix One of the most cryptic yet significant errors

Understanding the "Atomic Test-and-Set of Disk Block Returned False for Equality" Error

In traditional storage, locking a file required "SCSI Reservations," which locked an entire LUN (Logical Unit Number). This was inefficient. ATS allows for . Instead of locking the whole "parking lot," the system only locks a "single parking space" (a specific disk block). The process works like this: The VMkernel logs will fill with ATS Miscompare

Look for spikes in command latency. ATS is very sensitive to timing; if the storage is overloaded, ATS failures will increase.

The most common cause is that two different hosts are trying to access the same metadata at the exact same time. If Host A updates a block while Host B is still holding onto "old" information about that block, Host B’s next ATS command will fail because the block's state changed behind its back. 2. Storage Array Firmware Incompatibilities

When the system reports that this operation "returned false for equality," it means the phase failed.